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Overbite Clam

Photo: Andrew N. Cohen

Photo: Washington State University Beachwatchers

What are they?

Corbula amurensis, commonly known as the overbite clam, Asian marine clam or Amur River clam, is a tan, white or yellow clam, up to about 25 mm long. It lives partly buried in the sediment, with its hind third or half exposed above the surface. The right half of the shell is a bit larger than the left and slightly "overbites" it at the lower margin, which distinguishes the overbite clam from similar-looking clams on the Pacific Coast.

Are they here yet?

No, but they are present in San Fransisco Bay. The first record of overbite clams in North America was three specimens dredged from the bottom of San Francisco Bay in October 1986. By 1987, overbite clams had become the most abundant benthic organism in the northern part of the Bay, carpeting the bottom at densities of over 16,000 juvenile clams per square meter. Overbite clams most likely arrived in San Francisco Bay as larvae discharged from a ship's ballast tank.

Why should I care?

The overbite clam grows prolifically, forming dense quantities very rapidly and displacing other native species. They also consume large amounts of plankton, a major food source for critical fish species – such as the delta smelt – and other aquatic organisms, by sucking in and filtering plankton from the water. The larger the geographic distribution of the clam, the more widespread its effects on the aquatic food web.